DepthCharge

The surface of the ocean off Florida's upper keys is a sickly blue mottled with green and purple. A shallow reef, slightly out of focus, shimmers some twenty or thirty feet below. My companions - Pete, Susan, and James - and I slip overboard wearing ridiculously large fins: three feet long and stiff as boards. The divers slide under the surface, drifting downward like skydivers in slow motion.

The last time I had seen my friend Pete, he showed me a photograph of two pilot whales plummeting into an electric-blue abyss. During his crossing of the Gulf Stream from the Bahamas, more than a hundred whales had materialized from out of the afternoon sea glare, and when Pete saw them, he jumped off the boat to join them. Wearing only a mask, a snorkel, a pair of fins, and his Speedo, he was circled by a pod of seven pilots, each the size of a semi. He began to surface and descend in sync with the whales, and he followed them into their ever darker home, over a hundred feet down on the strength of his own lungs.

Though Pete felt drawn down further with the whales, he remembered an important fact: He was running out of time to surface. The first and strangest rule of free diving - as the extreme sport of plunging to scuba depth without air tanks is known - is that the further down you go, the less you think you need air, due to the effect of water pressure on the human body. Pete has passed out underwater four times, once landing in the hospital with an abscess the size of a golf ball in one lung.

"Not only do you want to stay underwater, but you don't feel the need for air," Pete says. "You never really feel the need for air until you're near the surface." The idea of plunging one hundred feet into the ocean and becoming so intoxicated you forget to return stayed with me. And Pete, after I'd pestered him for years, finally agreed to escort me below the surface of civilization. A congested desk worker, I dream of jumping into fathomless depths - what divers call blue water - and, like Pete, communing with deep-sea aliens.

Attempting to imitate Pete, James, and Susan, I dip my head beneath the surface and flail toward the reef below. I get maybe twenty feet when I feel a vice clamping down on my forehead. Pete looks puzzled when I quickly surface. He follows me up and tells me to take off my mask, and my hand comes away with a clot of blood: The pressure has tom the sinus tissue in my forehead. I have run afoul of the first physical requirement of free diving: a clear sinus.

Beyond that, the main athletic tools divers need are lung elasticity and comfort at depth, both won only by constant immersion in the sea. The trick is teaching the body to use air gradually under extreme stress, a largely mental skill that demands relaxing while using your muscles.

We dive in successively deeper water all morning, me trying gamely to work around my sinus problem without much success. Finally, around mid-afternoon, we reach blue water, where the floor begins to taper away into the Gulf Stream.

Blue water is the name for open sea without visible sides or bottom. No reefs, no visible shore, nothing for the mind to hold onto. The coast is about twelve miles away. In every direction the sea has the same calm and brightness of a swimming pool without walls. We jump in. Pete, James, and Susan are motionless. They are hyperventilating, flooding their systems with oxygen in preparation for the descent. Their breaths, amplified through the snorkels, become longer and slower, the sound of someone sinking into sleep.

Pete and James make a few easy drops to fifty and sixty feet, and then aim for real depth. The first ten feet down calls for kicks and muscle work. The divers slowly sink, head first, plugging their noses to equalize ear pressure. The weight of the ocean pressing down from above increases, and the divers begin to glide, gaining momentum fast and becoming negatively buoyant. Then they stop flippering, they have kicked free of the surface's pull. At fifty feet they become silhouettes, then, near sixty, ghostly shadows. Then they're gone. Pete focuses entirely on streamlining his body, arms forward, head tucked to avoid friction. At eighty feet, he can hear water rushing past him. Breaking out of the human missile position, he spreads his arms and legs and just falls downward. Following ten feet behind, James sees him dropping toward the bottom on his back with a euphoric grin on his face.

Pressure at these depths does funny things to your body. It reduces the heart rate and simultaneously increases the body's absorption of oxygen, lulling you into a peaceful torpor. Your original breath of surface air is compressed to a third of its initial volume, and the lungs begin to collapse, leaving a hollow, empty feeling in the chest. Some divers feel their ribs knocking together.

Yogic control over the mind and body, both of which are confused over whether to ascend or descend, becomes crucial. You not only have to control the natural tendency to panic, but, paradoxically, remain lucid enough to see through the free-diving high so you can remember to allow enough time to surface. To a diver in deep water - adrift on currents a hundred feet down, staring into the utterly silent sheet of blue, up and down looking eerily similar - the world can truly just stop making sense.

As your body decompresses on the way up, its rate of oxygen absorption returns to normal. The good feeling disappears, and the danger of shallow-water blackout looms. The huge surplus of air you feel you have at depth might actually turn out to be dangerously little near the surface. You can feel flooded with oxygen at sixty feet and yet suddenly pass out from lack of it at ten. Two or three divers faint and drown every year, blacking out just feet from air. Sometimes you feel it coming, sometimes you don't.

I watch as Pete, James, and Susan surface safely, their bodies undulating in S-shaped dolphin kicks. They rest motionless in the water in the dead-mans float, hands cupped together, so as to expend no energy on movement and to resaturate their bloodstreams with oxygen. Suddenly a shadow takes shape below and behind them. As it beelines for the huddle of divers, all I can do is point, speechless, for what seems like the longest second. Not until it stops next to their legs and stares directly at me do I realize it is a sled-size loggerhead turtle. It bolts when we try to take its picture.

I begin to look over my shoulder during the next dive, sensing other shapes just out of vision. The blue wall now seems impenetrable. Coming up from another dive minutes later, James shouts something through his snorkel, a hard word, either "Look!" or "Shark!" We turn, and hovering within what seems like arm's reach is a lance connected to an eyeball: the face of a sailfish the size of a human. A rigid mane, its dorsal fin, is fanned out, arched along its spine. It points its bill at me and then moves away without twitching a fin, folding its sail as it disappears into the blue shadows.

We climb back on the boat. "Maybe the love vibe is attracting all the creatures: says Susan. "Or the fear vibe: I say. Pete has a deep-water look: goofy, nitrogen-saturated, dazed. The sudden appearances of the two archaic creatures, with their beak and shell and bill, have made my skin crawl. They had been drawn to us, undeterred by scuba bubbles, following the divers up from the depths, their faces hinting at secrets from the floor of the ocean and then bored or startled or hungry, they left. Such visitations from the deep make me feel naked and ungainly.

The ocean has begun to work its creepy spell. James does not want to go back in, so he cracks a beer, a sign I ignore because I am determined to try one more dive. James reads visitations by fish like some people read coffee grounds, a habit acquired by necessity. On Labor Day, 1989, at three-thirty in the afternoon, he was hunting at Biscayne Bay. He stopped at a small reef, and was on his knees underwater trying to catch a lobster when the sun disappeared behind a sixteen-foot hammerhead. James slowly surfaced and swam toward the boat, eighty yards away, and the shark followed beneath him. The shark continued to make lazy figure eights, brushing past James and surveying him with each eye until he got to the boat.

Fear of sharks is a psychological barrier that every free diver - especially spear hunters, who draw blood - must face. Terry Maas, who holds the world record for spearing a 398 pound bluefin tuna, can't forget that his best friend bled to death after being bitten by a great white; he now carries an explosive bang stick on his dives. James did not hunt for seven months after his encounter with the hammerhead.

In the end, James decided that sharks are a "necessary implication" of a choice he has made to be "married to the ocean." For him the nearness of such predators is a sign that the ocean is still wild territory, one of the last places where you can literally enter the food chain and get a little closer to the animal in us. "I would rather die doing something like that," he says.

On a final dive, my fin barely dips below the surface when a shark materializes, angling up out of the deep. I see its black gills and dark gouge of an eye, its pointed face. Abandoning all shark protocol, I offer my legs to the demon and flee for the boat. Pete later jokes that the whole boat would have gotten to me faster than I to it if I had simply shouted. If free diving is about leveling the playing field and entering the web of life, I got what I came for, I suppose. There's nothing like coming face-to-face with a shark to make you feel alive.

I ask Pete if things always pop out like that in deep water. Yes, that's why he doesn't swim it more often, he says. "Every time I go out, something appears from behind that blue curtain."

Duncan Bock practices holding his breath in his bathtub in Brooklyn